1. Field of the Invention
The present invention relates generally to acousto-electric, or space-charge-enhanced, convolvers and more particularly to acousto-electric convolvers having a plurality of channels wherein the coupler means for sensing electric fields associated with counter-propagating surface acoustic waves is distributed over the plurality of channels.
2. Description of the Prior Art
The operation of acousto-electric, or space-charge-enhanced, convolvers may be based, for example, on the nonlinear interaction of electric fields generated by oppositely directed surface acoustic waves counter-propagating on a piezoelectric film or substrate with charge carriers in a layer or wafer of semiconductor material or on nonlinear signal mixing in a chain of semiconductor diodes.
Earlier devices of this type employ a piezoelectric substrate of lithium niobate and a wafer of silicon spaced above the acoustic beam, or propagation path for the acoustic waves, on the substrate. The air gap spacing between the piezoelectric substrate and the silicon wafer is on the order of 1000 angstroms. For a discussion of such devices see, for example, G. S. Kino, W. R. Shreve, and H. R. Gautier, "Parametric Interactions of Rayleigh Waves," 1972 Ultrasonic Symposium Proceedings, IEEE Cat. No. 72 CHO 708-8SU, pages 285-287 and J. M. Smith, E. Stern, and Abraham Bers, "Accumulation-Layer Surface-Wave Convolver," Electronics Letters, Vol. 9, No. 6, Mar. 22, 1973, pages 145-146. In the air-gap convolvers discussed in the above-referenced articles, the output signal is sensed across a pair of electrodes, one on the outer or top surface of the semiconductor wafer and one on the bottom or outer surface of the piezoelectric substrate.
A major disadvantage of air-gap convolvers is that a configuration which requires a semiconductor wafer to be precisely and uniformly spaced across an air gap from a piezoelectric substrate is difficult and expensive to manufacture in quantity to the tolerances required for consistent results. However, if the semiconductor wafer is placed in direct contact with the piezoelectric substrate, it interferes with the propagation of surface acoustic waves thereon.
Therefore, interest has focused on the utilization of arrays of interdigital transducer taps and on multistrip couplers. Multistrip couplers are arrays of parallel closely spaced narrow strips of conductor material disposed on the surface of a piezoelectric surface acoustic wave propagation medium perpendicular to the wave vectors. The multistrip coupler spans the interaction region, or propagation path, and extends outward therefrom. Such a multistrip coupler is described in F. G. Marshall and E. G. S. Paige, "Novel Acoustic-Surface-Wave Directional Coupler with Diverse Applications," Electronics Letters, Vol. 7, No. 16, Aug. 12, 1971, pages 460-462. A multistrip coupler may be used to couple the normal components of electric fields associated with surface acoustic waves propagating on a piezoelectric film on a substrate to an adjacent region of the same substrate or to a similar distinct substrate. A strip of semiconductor material can then be placed in contact with the multistrip coupler outside the acoustic beam without interfering with the surface acoustic waves. A multistrip coupled convolver having a silicon chip with a thin silicon dioxide coating of controlled thickness pressed into mechanical contact with the coupling strips of the multistrip coupler is described in W. R. Shreve and G. S. Kino, "Strip Coupled Acoustic Convolvers," 1973 Ultrasonics Symposium Proceedings, IEEE Cat. No. 73 CHO 807-9SU, pages 145-147.
While the mechanical contact model for a multistrip coupled convolver offers some advantages over air gap devices, the most promising possiblity for the utilization of the multistrip coupled concept for convolvers is in its application to monolithic structures where the piezoelectric and semiconductor media are films deposited side by side on the same crystalline substrate. This approach has the advantage of enabling the fabrication of convolver devices to close tolerances in production quantities by the well established and relatively inexpensive lithographic techniques currently in use for the manufacture of integrated circuits.
Monolithic multistrip coupled convolvers are discussed by the inventor of the present invention in L. R. Adkins, "Strip Coupled AlN and Si on Sapphire Convolvers," 1973 Ultrasonics Symposium Proceedings, IEEE Cat. No. 73 CHO 857-8SU, pages 148-151 and in L. R. Adkins, "Monolithic Aluminum Nitride/Silicon-on-Sapphire Strip Coupled Convolvers," 1974 Ultrasonics Symposium Proceedings, IEEE Cat. No. 74 CHO 896-1SU, pages 228-231. A use for a multistrip coupled convolver is disclosed in "Monolithic Acousto-electric Image Pick-up Device", U.S. Pat. No. 3,970,778, issued to L. R. Adkins on July 20, 1976, and assigned to the assignee of the present application.
The use of an array of interdigital transducer taps distributed along the interaction region, or propagation path, of a surface acoustic wave delay line and connected individually to parallel or series connected diodes for nonlinear signal mixing is illustrated in Reeder et al, "Diode Coupled Tapped Acoustic Delay Line Correlator and Convolver," U.S. Pat. No. 4,016,514 issued Apr. 5, 1977.
Throughout this specification, the use of the terms "coupler means" and "coupler", unless further limited, are intended to refer equally to arrays of interdigital transducers, to multistrip couplers and to their equivalents.
A comparison of the performance of several types of convolvers is given in O. W. Otto, "Theoretical Comparison of Space-Charge-Enhanced Acoustic Surface Wave Convolvers," Proceedings of Symposium on Optical and Acoustical Micro-Electronics, New York, Polytechnic Press. 1975, pp 511-528. Acoustic surface wave convolvers having space-charge-enhancement means therein include combined media convolvers, separated media convolvers, strip coupled convolvers, and parallel and series diode convolvers.
A principal figure of merit for a convolver is its time-bandwidth product (TB). For signal processing applications, it is desirable to have TB as large as is practicable because TB is a measure of the amount of information that a device or system can process. It will be apparent to those skilled in the art that, in a surface acoustic wave convolver, TB is proportional to the distance between the surface acoustic wave input transducers or, equivalently, to the length of the propagation path for counter-propagating surface acoustic waves. The greater is this distance, the greater will be TB. However, in a prior-art single-channel convolver, wherein coupling means such as an array of interdigital transducer taps or a multistrip coupler is disposed over substantially all of the propagation path, the propagation path cannot be extended indefinitely without incurring penalties.
If the propagation path is lengthened, and thus the coupling means is lengthened also, problems may arise because of the presence of an increased amount of conductor material overlying the piezoelectric material. Unwanted dispersion and reflection, or scatterings, effects are increased by this additional conductor material. Losses are also increased.
In addition, a requirement for the fabrication of a single-channel coupler extending over a relatively long propagation path is likely to exceed the upper size limitations of currently available lithographic fabrication facilities and techniques. The development of contemporary lithography has been pressed by a desire to make electronic devices smaller, not larger